This invention relates to a prosthetic acetabular cup and to a prosthetic femoral joint incorporating such a cup and comprising a prosthetic femoral stem element which includes a part-spherical bearing head and in which the acetabular cup is adapted to be secured in an acetabulum.
Prosthetic femoral joints of the kind set forth above are well known and the range of angular movement between the bearing head and cooperating acetabular cup is limited by the femoral stem which extends from the bearing head engaging the outer peripheral rim of the cup which is usually in a single plane although in some constructions one side of the peripheral rim may be slightly higher than the other.
The peripheral rim of a natural acetabulum is however contoured in relation to a theoretical plane, for example the proximal shape of the rim is lower between the illium and the pubis, between the pubis and the ischium and between the ischium and the illium. The contour between the pubis and the ischium is lower than the contour between the pubis and illium and between the ischium and the illium. Again, the contours of the rim in a distal direction are higher at the ischium and the illium than the remainder of the rim. Due to the contoured shape the angle of movement at certain positions of the natural femur is greater than that provided by a convention prosthetic femoral joint.
A further difficulty with known prosthetic cups is that if they are not correctly positioned in the acetabulum, or the cup is slightly too large, part of the rim of the cup can be outside the acetabulum in which it is fitted and this can cause the patient pain due to the rim conflicting with the psoas muscle which extends from the acetabulum to the femur, or other muscles or tissues.